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CHAPTER 19 



noninverted one, being heterozygous for the 

 inversion. If the inversion is very small the 

 homologs will pair properly everywhere but 

 in the inverted region. Here, because the 

 homologs cannot, in so short a region, twist 

 enough to make homologous loci meet, they 

 will fail to synapse, no chiasma will be formed 

 and no crossing over will occur. Insofar as 

 crossing over can lead to more adaptive 

 recombinants, such inversion heterozygotes 

 are at a disadvantage from this standpoint, 

 as compared to noninversion or inversion 

 homozygotes, because of the absence of re- 

 combination among genes within the inverted 

 region. Nevertheless, very small inversions 

 may survive in any species. 



But consider now the meiotic behavior of 

 heterozygotes for larger paracentric inver- 

 sions. In this case (Figure 19-3 A), synapsis 

 between homologs is made possible in all 

 regions, with the exception of the parts near 

 the points of breakage, by one homolog 

 looping in the inverted region while the other 

 does not. It happens that this Figure shows 

 the inverted chromosome looping, but the 

 reverse is equally likely to occur. Note that 

 only two nonsister strands are shown; the 

 other two naturally do not have a chiasma 

 where these two have formed one. If one or 

 more chiasmata occur outside the inverted 

 region the four meiotic products will each be 

 eucentric (having one centromere), as usual. 

 If, however, one chiasma occurs anywhere 

 within the inverted region, as shown between 

 C and D, two strands of the tetrad will be 

 eucentric (one with and one without the in- 

 version, being those strands of the tetrad not 

 shown in the Figure) and two will be aneu- 

 centric (having no centromere or more than 

 one). One of the aneucentrics will be acentric 

 (duplicated for A and deficient for G.HIJ) 

 while the other will be dicentric (having 

 duplicated and deficient segments that are 

 the complement of the acentric's). If the 

 inversion is only moderately long, only one 

 chiasma may occur within it, but if it is 



sufficiently large, some 2-strand double chias- 

 mata may occur within it, in which case the 

 crossover strands will be eucentric. 



Regardless of the length of a paracentric 

 inversion, a single chiasma within the inverted 

 region in an inversion heterozygote will pro- 

 duce two aneucentric, aneuploid meiotic prod- 

 ucts, as we have just seen. In animals that 

 undergo crossing over each of these products 

 will enter a gamete, which will function but 

 usually have a dominant lethal effect after 

 fertilization. This means that such individ- 

 uals are at reproductive disadvantage, and 

 this often leads to the elimination of the 

 inversion from the population soon after it 

 arises as a mutant. In certain species where 

 there is no crossing over in one sex (the 

 Drosophila male, for example), any homolog, 

 inverted or not, has the same chance of being 

 included in the gametes produced by that sex. 

 There is a special factor to consider, however, 

 with regard to meiosis in the Drosophila fe- 

 male (the sex in which crossing over does 

 occur), where it has been found that the two 

 meiotic divisions occur in tandem, just as in 

 Neurospora (see p. 123). In the Drosophila 

 oocyte heterozygous for a paracentric inver- 

 sion, a single chiasma within the inverted 

 region produces a dicentric at anaphase I 

 which orients the dyads at metaphase II so 

 that the two eucentric monads proceed to the 

 outermost of the four poles at anaphase II. 

 At the end of telophase II, the four meiotic 

 products, therefore, are arranged in a row: 

 eucentric, part of dicentric, remainder of 

 dicentric, eucentric — one of the two end 

 eucentric-containing nuclei becoming the egg 

 nucleus, the others degenerating. In this way 

 the dicentric strand is shunted away from the 

 egg nucleus, which therefore receives one of 

 the two eucentric, noncrossover strands. In 

 Drosophila, therefore, paracentric inversions 

 of any size rarely cause aneuploid gametes 

 in either sex and can become established in 

 nature. Note, however, that in this species, 

 the production of crossover-containing 



